(1) Field of the Invention
This invention relates to the local manipulation of electromagnetic fields, and more particularly, but not exclusively, to the use of switchable devices to allow RF (radio frequency) tags to be mounted in proximity to a metallic surface, and to be activated or deactivated as required.
(2) Description of the Art
Electronic devices which can interact with incident electromagnetic radiation, and emit a signal that can be detected by an appropriate reader are widely used for the identification and tracking of items, particularly for articles in a shop or warehouse environment. Such devices, which will be referred to throughout this specification as EM tags or RFID tags or simply as tags, usually comprise a chip coupled to an integral antenna which is tuned to a particular frequency of operation. The frequency of operation of current EM tags is generally radio frequencies (RF), including the ultra high frequency (UHF) and microwave ranges but the present invention is applicable to a tag which operates at any frequency. The tags may be passive, in that they interact with incident radiation of the appropriate frequency and re-transmit a modulated signal back to a reader, or active in which the tag contains its own power source.
One commonly experienced disadvantage with such tags, especially passive tags, is that if directly placed on (or within a several millimeters of) a metal surface their read range is decreased to unacceptable levels and—more typically—the tag cannot be read or interrogated. This is because a propagating-wave RF tag uses an integral antenna to receive the incident radiation: the antenna's dimensions and geometry dictate the frequency at which it resonates, and hence tailor the frequency of operation of the tag (typically 866 MHz or 915 MHz for a UHF (ultra-high frequency) range tag and 2.4-2.5 GHz or 5.8 GHz for a microwave-range tag). When the tag is placed near or in direct contact with a metallic surface, the tag's conductive antenna interacts with that surface, and hence its resonant properties are degraded or—more typically—negated. Therefore the tracking of metal articles such as cages or containers is very difficult to achieve with UHF RF tags and so other more expensive location systems have to be employed, such as GPS.
UHF RFID tags also experience similar problems when applied to certain other surfaces which interact with RF (radio frequency) electromagnetic waves, such as, certain types of glass and surfaces which possess significant water content, examples including certain types of wood with a high water or sap content. Problems will also be encountered when tagging materials which contain/house water such as, for example, water bottles, drinks cans or human bodies etc.
One way around this problem is to place a foam spacer, or mounting between the RF tag and the surface, preventing interaction of the antenna and the surface. With currently-available systems the foam spacer needs to be at least 10-15 mm thick in order to physically distance the RF tag from the surface by a sufficient amount. Clearly, a spacer of this thickness is impractical for many applications and is prone to being accidentally knocked and damaged.
Other methods have involved providing unique patterned antennas which have been designed to impedance match a particular RF tag with a particular environment.
Visual identifiers such as barcodes are currently employed by the retail and logistics industry to track and trace goods, ranging in size from containers and pallets to individual items. A disadvantage of barcodes is that they require line of sight in order to be interrogated. RFID tags have largely overcome this problem in that they can function without direct line of sight provided that there is RF contact. However, one of the problems with RFID is that after the item has been removed from the controlled area such as a store or depot, the RFID tag is still operable, typically at distances in the region of 3 to 10 meters depending on the interrogating equipment and RF tag being employed. This may be desirable for items which are to be shipped from one location to another. However, there is increased concern over the privacy issue for the consumer that their products are still traceable even after they have bought the item. Hence a person could effectively be scanned unwittingly and their purchases etc could be determined. Further privacy concerns are that the RFID tags could be used to trace the whereabouts of a person. Similarly, in a stock control application, it may be desired to exclude damaged or spoiled stock, even though such stock is still fitted with an RFID device.
RFID chip manufacturers have recently attempted to overcome this problem by using a disable function, such that once the RF tag is read the interrogating device provides a kill command to deactivate the tag. This method provides several drawbacks: firstly if the tag is permanently inoperable then refunds at a shop or reactivation to find out product information will be unduly complicated. If the kill command can be reversed by a secure wake-up command, then this security could be easily bypassed or the codes overcome. Furthermore changes within the chip may be undetectable to the user.